Effect of irregular wave-induced pore pressure on seabed scour development around a verticle slender pile
This Masters’ thesis reports on a large scale investigation of scour development around a slender pile under irregular JONSWAP spectrum wave loading to better understand the processes behind scour development and to determine if bed pore pressure response influences scour development. A thorough background review of relevant scour-related topics is first presented. The scour dimensions and the pore pressure response in the sediment are functions of the wave climate, the average sediment grain size diameter, the relative density of the sediment and the internal friction angle. To the knowledge of the author, there are currently no directly applicable computational or empirical models for the specific case of sediment pore pressure response under irregular wave loading. Physical properties of the fine sand used in the sediment bed were determined through a series of laboratory tests. There was some agreement between measured pore pressure response and an analytical model generated by Ulker et al. (2009). The model was used to estimate sediment saturation and to visualize the affects of different parameters in pore pressure response. Water surface elevation, pore pressure response, depth of scour, pile vibrations, and bed velocities were compared near the pile and in the far field using the time series, frequency response spectra, and time series of specific frequency components. It was determined that observed pore pressure trends were not influencing scour development but were rather a result of the scour development and the water surface elevation. In particular, the pore pressure transducers exhibited larger fluctuations in pore pressure (both negative and positive) than for individual wave events alone. These pore pressures are thought to result from long period waves. As the scour near the pile deepened, the overburden of the transducers was decreased and the damping of the pressure response with depth was likewise reduced. There appeared to be no influence of the measured pore pressure on scour development. Scour development was observed to be directly related to bed velocities resulting from individual passing waves. Long term trends in scour were observed to be related to ranges of bed velocities. Scour depth generally increased during time ranges of low bed velocities, stayed the same during medium velocities, and decreased during large bed velocities. This observed relationship appears to be counter-intuitive but can be explained by the increased live bed scour surrounding the pile during larger bed velocities. More live bed scour would create more suspended sediment in the water column so that between waves new sand, which originated from the surrounding bed, would be deposited in the scour hole. The entire flow regime is highly turbulent and as such there is not a direct relationship of increasing bed velocity resulting in increased rates of scour. Instead, the high turbulence during wave energy spectrum 4 causes large variably in that relationship due to the complex flow patterns and strong vortices which evolve. The time ranges of high bed velocities affecting scour are not a direct function of the water surface elevation due to the high turbulence around the pile and as such the bed velocities were looked at separately from the water surface elevation for trends in scour development. These results show that, even in the very large wave flumes, the depth of the sediment is a significant limiting factor in understanding the pore pressure response for in-situ conditions. The pore pressure response was not observed to have influenced the development of scour for the tests conducted. As such, the scale effects of a finite sediment bed depth and pore pressure response will probably not affect the investigation of time rate of scour or depth of scour developed. The scale effects of a finite seabed depth in model testing should be investigated further in future model tests where the possibility exists for residual pore pressures. (Abstract shortened by UMI.)
Kevyn A Bollinger,
"Effect of irregular wave-induced pore pressure on seabed scour development around a verticle slender pile"
Dissertations and Master's Theses (Campus Access).